Back to the original topic, though I don't believe backwards time travel is possible, it would not directly violate the conservation of mass/energy law. It has to do with the reference frame used. Though an observer from the past might see an object instantly pop out of nowhere from the future, which from their reference frame appears to violate the conservation laws, from the reference frame of the object traveling through time, no laws are violated.
This is the same reasoning for why distant objects from earth can appear to be moving faster than the speed of light, but relative to the objects' reference frame, the speed of light is not being broken. When relativity is taken into account, the laws often appear broken from other reference frames but are still conserved locally.